Optical transceivers are key components in optical fiber-based telecommunications and data communication networks. An optical transceiver includes an opto-electronic light source, such as a laser, and an opto-electronic light receiver, such as a photodiode, and may also include various electronic circuitry associated with the laser and photodiode. For example, driver circuitry can be included for driving the laser in response to electronic signals received from the electronic system. Receiver circuitry can be included for processing the signals produced by the photodiode and providing output signals to the electronic system. Optical lenses are also commonly included.
Generally, there are two types of semiconductor laser devices: edge-emitting lasers and Vertical Cavity Surface Emitting Lasers (VCSELs). An advantage of VCSELs is that they can be tested economically at wafer level rather than chip level. Another advantage of VCSELs is their well-defined spot size, which promotes high coupling efficiency to optical fibers without the need to provide beam shape correction, thus facilitating economical packaging. Edge-emitting lasers also have advantages, such as robust reliability and high output optical power. Likely for these reasons, edge-emitting lasers remain the most commonly used semiconductor laser in high-speed optical transceivers. To test edge-emitting lasers, a wafer must be scribed and cleaved for single-chip testing. That is, wafers must be diced into bars, coated with highly-reflective (HR) or anti-reflective (AR) coatings, and then diced to single chips to be tested and selected. The process of testing edge-emitting lasers thus can be relatively uneconomical.
One way to reduce edge-emitting laser chip cost involves a process commonly referred to as etched facet. In an etched facet laser, the feedback mirrors are etched facets rather than cleaved facets. Etched facets facilitate coating the facet with HR or AR layers at wafer-level, rather than at bar level. (See, e.g., Peter Vettiger, et al., IEEE Journal of Quantum Electronics, Vol. 27(6), June 1991, p. 1319.)
Edge-emitting lasers can have either a ridge waveguide structure or a buried waveguide structure. The process of fabricating a ridge waveguide structure is less complex than the process of fabricating a buried waveguide structure. For uncooled lasers, the active core layers are commonly made of aluminum-containing multiple quantum well (MQW) layers. In such a laser, a ridge waveguide structure is more advantageous than a buried waveguide structure because a ridge waveguide structure does not have an etched MQW region or suffer from aluminum oxidation.
As illustrated in FIGS. 1-3 (not to scale), a known ridge waveguide laser structure 10 includes etched-facet windows 12 fabricated directly on the structure using a protection mask (not shown). The term “window” refers to a region etched down from the surface to the substrate. The facets defined by windows 12 are coated with either an HR or AR coating material, depending on whether the resulting structure is to be a Fabry-Perot (FP) laser or Distributed Feedback (DFB) laser. (For purposes of clarity, the relatively thin coating is not separately illustrated in the drawings.)
Multi-mask layers (not shown) are commonly used to fabricate a structure such as ridge structure 14. The first mask, which can be a Benzocyclobuten (BCB) layer, is used with an etch-back process to planarize the ridge. Then, an SiO2 mask is deposited on the BCB surface followed by photolithography and an SiO2 dry-etch process. An inductively coupled plasma (ICP) etch process is then performed to form windows 12. Lastly, the SiO2 and BCB layers are removed.
Additional fabrication steps can be performed on the resulting structure 10. A common additional step involves applying a photoresist material. Such photoresist material tends to flow into the relatively deep windows 12 and pool or accumulate there to a greater extent than in the mid-portion of the structure approximately mid-way between windows 12. As illustrated in FIG. 3, this effect causes the photoresist layer 16 to be thicker in the mid-portion than nearer to windows 12. (It can be noted that as multiple structures 10 are commonly fabricated together in end-to-end arrangements on a semiconductor wafer, two adjacent structures 10 share the window 12 in which the photoresist material pools.) Such non-uniform thickness of photoresist layer 16 can hamper further steps in the device fabrication process. It would be desirable to provide an etched-facet semiconductor laser or integrated device that poses fewer fabrication challenges than prior devices of this type.